A recently developed vacuum tube for handling r.f. signals includes a cathode for emitting a linear electron beam, a grid positioned parallel and in close proximity to the cathode (no farther than the distance an emitted electron can reach in a quarter cycle of a signal being handled by the tube) for current modulating the beam, and a cavity resonant to the frequency of the signal positioned between the grid and a collector electrode for the beam. The grid is coupled by a structure resonant to the frequency being handled by the tube to an input of the tube. Very high efficiency is achieved with such a tube by biasing the grid so current flowing from the cathode toward the grid occurs for no more than one half cycle of the r.f. signal handled by the tube. The grid is formed of a non-electron emissive material, such as pyrolytic graphite or molybdenum.
In one prior art configuration, a resonant input circuit supplies electric fields in opposing phase between the cathode and grid and between the grid and an accelerating anode positioned between the grid and an output cavity. In another prior art device, a second resonant cavity positioned between the output cavity and the accelerating anode is adjusted so the resonant frequency thereof is above the frequency being handled by the tube, to increase the average efficiency of the tube. These prior art structures are disclosed in the commonly assigned U.S. Pat. Nos. 4,480,210, 4,527,091 and 4,611,149. Commonly assigned patent applications generally dealing with similar tubes are Lien et al., Ser. No. 07/508,442, filed Apr. 13, 1990, now U.S. Pat. No. 5,317,233, and Lien, Ser. No. 07/508,611, also filed Apr. 13, 1990, now U.S. Pat. No. 5,233,269.
Commercially available tubes of this type have included a resonant structure for coupling the input signal to the cathode-grid assembly in the form of a resonant cavity coaxial with the cathode and the electron beam emitted from it. This resonant cavity has a length in the direction of the beam axis that is nominally either a half or full wavelength at the frequency handled by the tube. In practice, it is most usually at the full wavelength of the frequency handled by the tube causing the tube to have a relatively long length. The input signal to the cavity is capacitive-coupled to the cavity. A metal structure in the input resonant cavity couples the field established in the cavity in response to the input signal to the grid. An r.f. electric field is thereby established between the grid and cathode, to current-modulate the electron beam. An r.f. field is also established in opposing phase between the grid and anode.
Regeneration and increased gain are obtained in the prior art tubes by energy transfer between a pre-bunched beam and an r.f. field in the grid-anode space. To achieve this regeneration and increased gain, a driver circuit for the prior art tubes becomes electrically complex and difficult to design. Considerable time and effort for empirical design of the driver circuit and tube are necessary to achieve the desired results. It is difficult to adjust the driver cavity and tube parameters to achieve the optimum relative intensity and phase relation of the electric fields in the two r.f.-field regions. It is usually necessary to provide numerous tuning stubs and/or other variable resonant structures to provide the optimum relation.
Electrons leaving the grid and accelerated toward the anode are bunched while traversing an interaction region between the grid and cathode. Any impedance presented to the electrons by either free space or resonant modes in surrounding metal or dielectric containers causes r.f. radiation and/or oscillation. This reduces the tube power gain or interferes with other equipment. Previously this problem was handled by reducing the r.f. grid-anode gap impedance substantially to zero by bypassing it with a blocking capacitor or by connecting the grid-anode gap to low impedance coaxial or strip line open-ended resonant by-pass circuits. Whatever approach is taken, full beam voltage, e.g. 32 kV or 85 kV, appears across the grid-anode gap and must be considered, as must the r.f. voltage. The blocking capacitor or by-pass circuit must be in a potting compound to minimize and preferably eliminate high voltage, D.C. arcing.
There are several disadvantages in connecting the blocking capacitor or by-pass circuit between the grid and anode. Potting high voltage capacitors and other types of by-pass circuits capable of handling 32 or 85 kV is a problem; reliable arc-free operation is difficult to obtain. In addition, power gain is reduced because the potting compound is lossy. While tuning the grid-anode gap with open resonant lines makes voltage isolation relatively easy, such structures require extra space, tuning procedure and controls.
It is accordingly an object of the present invention to provide a new and improved electron beam vacuum tube including closely spaced cathode and non-emissive grid electrodes employing a relatively simple resonant structure for coupling an r.f. signal between these electrodes.
Another object of the present invention is to provide a new and improved electron beam vacuum tube including closely spaced cathode and non-emissive grid electrodes having an improved structure for reducing r.f. fields in a gap between the grid and a high voltage accelerating anode.
An additional object is to provide a new and improved electron beam vacuum tube including closely spaced cathode and non-emissive grid electrodes that is easily tuned over a wide frequency range, e.g., the U.H.F. spectrum.
A further object is to provide a new and improved input coupling structure for electron beam vacuum tubes including closely spaced cathode and non-emissive grid electrodes.
An added object of the present invention is to provide a new and improved electron beam vacuum tube including closely spaced cathode and non-emissive grid electrodes having an improved structure for minimizing r.f. coupling to leads for supplying grid bias, cathode bias and heater current to the tube.